I've had the same problem when dealing with a dodgy upgrade, but you have the postfix folder that sudoers is looking for. I guess you could duplicate the line and change the path so it has rights for both, but that wont deal with the underlying issue.

I dont have the final answer I'm afraid; stick around, I'm sure someone will come up with something.

I tried upgrading from 4.5.6 to 4.5.11 then to 5.0.2
I had the same mta password request.

Then I found this comment about sunsets:

"You MUST update the server OS on 4.5.x before you begin the upgrade to ZCS 5.0. To do this you must first upgrade from 4.5.x to 4.5.10 (which has the current server OS) and THEN you can safely run an upgrade to ZCS 5.0. Not doing so will fail the install."